A network service that identifies all resources on a network and makes them accessible to users and applications. Resources include e-mail addresses, computers, and peripheral devices such as printers. Ideally, the directory service should make the physical network topology and protocols transparent so that a user on a network can access any resource without knowing where or how it is physically connected.

There are a number of directory services that are used widely. Two of the most important ones are LDAP, which is used primarily for e-mail addresses, and Netware Directory Service (NDS), which is used on Novell Netware networks. Virtually all directory services are based on the X.500ITU standard, although the standard is so large and complex that no vendor complies with it fully.